Technical Field
The present invention relates to a process and a catalyst for converting methanol into light olefins. More specifically, the process and the catalyst increase the selectivity of the methanol conversion to propylene.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Increasing cost and uncertain supply of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of methanol into light olefins such as ethylene, propylene and butylenes. There is a specific interest in the use of methanol to produce light olefins due to the fact that methanol is being produced worldwide in large quantity from natural gas through the production of synthesis gas (mixture of CO and H2) from methane gas which is then converted to methanol. Methanol can also be produced from coal and biomass processing. Methanol is converted commercially into propylene using randomly packed pellet catalysts. This process has limitations, such as lower selectivity to propylene and higher yields of byproducts such as C2, C4 and C5+ olefins and paraffinic hydrocarbons.
Processes for converting methanol to light olefins are well known in the prior art. Early catalysts used for this conversion reaction were based on aluminosilicates molecular sieves. These processes have been described in U.S. Pat. Nos. 4,238,631, 4,328,384, 4,423,274 and 4,499,327 (each incorporated herein by reference in its entirety). These patents reveal the deposition of coke onto the molecular sieves in order to increase selectivity to light olefins and minimize the formation of C5 and higher hydrocarbons (C5+) hydrocarbons as byproducts. The effect of the coke is to reduce the effective pore diameter of the molecular sieves. The prior art also disclose that silicoaluminophosphates molecular sieves can be used to catalyze the methanol to olefin process.
Propylene is perhaps one of the oldest and most important of the crucial building blocks of the petrochemical industry and one of the principal light olefins. From propylene, important industrial derivatives such as polypropylene, acrylonitrile, propylene oxide, 2-propanol, cumene/phenol, oxo-alcohols, isopropanol, acrylic acids, and oligomers are obtained. Various additional products use propylene as a feedstock. Hence, its use can be seen to span a wide span of end-use industries, from automotive and construction, to polymers, consumer durables, packaging, medical, and electronics.
Historically, propylene was readily available, either as a co-product of heavy liquids cracking or from refinery sources. Growth in demand for propylene derivatives has outpaced that for ethylene derivatives for several years. The higher propylene demand has largely absorbed readily available sources of propylene to yield, until now, a fairly balanced global market in terms of propylene supply and demand. However, an interesting dynamic is now unfolding in the United States whereby large amounts of natural gas from shale and other sources are being produced with their accompanying natural gas liquids (NGLs) such as ethane, propane and butanes. This additional NGL is being utilized in higher percentages in steam crackers, which in turn, is lowering available propylene supplies and changing the competitiveness of the North American ethylene chain.
With the recent discovery of U.S. shale gas reserves and the increase in ethane cracking currently taking place that is set to only increase exponentially in coming years, U.S. propylene supply tightened by large amounts. This is because the cracking of light feedstocks produces dramatically less propylene co-product than the cracking of heavy liquids. Consequently, propylene production in the United States from ethylene crackers has declined, and, for the first time in 20 years, propylene prices were higher than ethylene prices. This propylene supply/demand gap is projected to considerably widen in the next few years as propylene demand rises and even greater volumes of lighter feedstocks available from shale gas deposits rapidly replace heavy liquids in crackers. LPG cracking in Europe will have a similar impact, although the displacement of hydrocarbon liquids will not be nearly as pronounced as in North America. Hence, shortages of propylene feedstock are likely in these two regions—imbalances which will extend to other regions via higher propylene pricing.
In view of the foregoing, there exists a considerable need for new processes and catalysts for methanol conversions to light olefins with improved selectivity towards propylene and also preferably a lower coking of the catalyst.